Assessment of the Erosive Wear Kinetics of Epoxy Coatings Modified with Nanofillers

doi:10.4236/jsemat.2013.31A013

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Journal of Surface Engineered Materials and Advanced Technology, 2013, 3, 89-93

http://dx.doi.org/10.4236/jsemat.2013.31A013 Published Online February 2013 (http://www.scirp.org/journal/jsemat)

Assessment of the Erosive Wear Kinetics of Epoxy

Coatings Modified with Nanofillers

Danuta Kotnarowska*, Michał Przerwa

Department of Mechanical Engineering, University of Technology and Humanities, Radom, Poland.

Email: *[email protected]

Received October 20th, 2012; revised November 25th, 2012; accepted December 3rd, 2012

ABSTRACT

The paper presents results of investigation on the erosive wear kinetics of epoxy coatings modified with alumina or sil-

ica nanoparticles. Natural weathering caused a decrease of their erosive wear resistance. After a 3-year natural weather-

ing, highest erosive wear resistance showed the epoxy coating modified with alumina nanoparticles.

Keywords: Polymer Coating; Nanofillers; Erosive Wear Resistance; Natural Weathering

1. Introduction

During their service life, polymer coatings are exposed to

various environmental factors that deteriorate their pro-

tective and decorative functions. Among the exposure

factors that affect polymer coatings applied on technical

objects exposed to natural weathering conditions the fol-

lowing ones are most common: climatic factors (ultra-

violet radiation, heat, humidity), aggressive media and

erosive particles [1].

Erosion is the dominating wear process of polymer

protective coatings of agricultural, constructional and

mining equipment or facilities and occurs as a result of

erosive particle impacts typical for a given environment.

The erosive wear process occurs when hard particles

(mineral particles, sand, dust, soil lumps, hail etc.) im-

pact the surface of the object. Hard particles cause wear

of the surface which leads to material losses in the super-

ficial layer [2,3,4]. Erosive wear of polymer coatings

manifests itself in abrasion, peeling, scratching, losses of

their parts as well as deformation of the coating with

substrate—in the places where erosive particles impact

the coating. This is a complex process and its kinetics

depends on particles material kind, their geometrical pa-

rameters, shape, velocity and impact angle as well as on

the coefficient of friction between the coating and ero-

sive particle. Erosive wear intensity of coatings rises with

a synergetic action of heat strokes, ultraviolet radiation as

well as aggressive media. Environmental factors affect

elastic, frictional and strength properties of polymer

coatings determining their erosive wear resistance.

Ultraviolet radiation is the most dangerous climatic

factor that causes chemical and physical destruction of

polymer coatings [5,6] which, first of all, destroys super-

ficial layers generating silver cracks. Superficial silver

cracks can also propagate inside the coating. As the re-

sult of the action of ultraviolet radiation, an intensive

chemical and physical destruction of coating surfaces

occurs which, first of all, deteriorates coating decorative

properties [7]. UV radiation induces oxidation of coating

superficial layers what results in an increased brittleness.

This manifests itself in a loss of cohesion between bind-

ing resin and fillers and pigments. In the next stage, pig-

ment and filler particles are released from the coating

surface layers. This process is called chalking and causes

an increase of the surface roughness which, in turn,

causes gloss loss and colour fading deteriorating, ipso

facto, coating decorative properties. Moreover, micro-

roughness cavities create favourable environment for the

growth of microorganisms (such as viruses, bacteria and

mould fungi) leading to the coating degradation in result

of biological corrosion development, for instance in form

of etching pits in the coating structure. Pits generated in

coatings can develop and, in the final stage, they can

reach the substrate and due to this the coating loses its

decorative and protective properties. Deterioration of

decorative properties may also result from the action of

such aggressive media as: brine, marine breeze, acid

rains, exploitation fluids and bird droppings [8-11]. Ag-

gressive media induce also coating cracking and swelling

as well as generate pores [12,13] that deteriorate the pro-

tective effectiveness of the coating. Moreover, aggressive

media trapped beneath the coating, where they accumu-

late, may induce blistering processes, whereas, a direct

contact of aggressive media with the metal substrate

*Corresponding author.

Assessment of the Erosive Wear Kinetics of Epoxy Coatings Modified with Nanofillers

causes the development of corrosion processes. Corro-

sion products formed at the substrate surface reduce or

destroy the coating/metal bond what leads to the adhe-

sion loss [8,10,14].

The need to investigate kinetics of organic coating

erosive wear caused by hard particles impacting results

from the fact that the erosive wear process is not suffi-

ciently recognised. Additionally, in the studies on the

erosive wear kinetics of polymer coatings, the extent of

coating destruction induced by environmental factors

(aggresive media, ultraviolet radiation, mechanical stresses)

should also be taken into consideration because the ero-

sive wear intensity is conditioned by changeable envi-

ronmental conditions [15-17].

2. Method of Coating Sample Preparation

Three-layer epoxy coatings (Figure 1) were applied by

air-spraying on the steel substrate. The coating consisted

of two primer layers (2 and 3) and one unmodified (A) or

nanofiller-modified (B) surface layer.

The samples used in the investigation were made of

S235JR steel and were of dimensions 160 × 80 × 2 mm.

The surfaces of the samples were prepared by grit blast-

ing (in a special tumbler) with the use of small ceramic

bars. Before coating application the samples were de-

greased.

The epoxy paint was modified with alumina nanopar-

ticles (Al2O3) with the grain size of 20 nm or silica

nanoparticles with the size of 12 nm. The nanofiller con-

tent was 3.5% (by weight). The obtained coatings were

acclimatised for 10 days at the temperature of 20˚C and

the relative humidity of 65% ± 5% (PN-EN 23270:1993).

3. Investigation Methodology

The coating thickness was measured with the use of

Figure 1. Cross-section scheme of the investigated epoxy

coatings; (a) unmodified coating; (b) modified coating, 1—

steel substrate; 2—primer layer I (epoxide); 3—primer

layer II (epoxide); A—unmodified surface layer; B—nano-

fillermodified surface layer.

Mega-Check FE meter (according to the Polish standard

PN-EN ISO 2808:2000). The average thickness of the

three-layer coatings was equal to 120 μm, including the

surface layer with the thickness of 40 μm. The coating

hardnes was determined with the application of the

Buchholz method (according to the Polish standard PN-

EN ISO 2815:2004). For coating roughness measure-

ments the Hommel T500 tester was applied (according to

the Polish standards PN-87/M-04251, PN-ISO 8501-1:1996,

PN-ISO 8501-1:1998).

Methodology of the Erosive Wear

Resistance Evaluation

The resistance to erosive wear was evaluated using the

falling abrasive particles method which consists in sub-

jecting coatings to the action of abrasive material stream.

The investigation on the coating erosive wear was carried

out using the apparatus presented in Figure 2.

The resistance to erosive wear of polymer coatings was

evaluated applying the criterion S according to Equation

(1). It expresses a proportion of the total mass M of

erosive particles which erode the coating (exposing the

steel substrate surface of the ellipsoid shape with the

minor diameter d = 3.6 ± 0.1 mm) to the coating thickness

 (1)



200



915



Figure 2. Apparatus for erosive wear of polymer coating

testing: 1—container for erosive material; 2—pipe trans-

porting erosive material; 3—optical microscope; 4—tilting

holder for fixing metallic test specimen with examined

coating; 5—container collecting erosive material after the

test.

Assessment of the Erosive Wear Kinetics of Epoxy Coatings Modified with Nanofillers 91

where: S—erosive wear resistance, [kg/μm]; M—mass of

erosive particles, [kg]; G—average coating thickness,

[μm]. Particles of granulated alundum 99A (according to

the Polish Standard PN-76/M-59111) of grain number 30

(according to PN-ISO 8486-2) were used as abrasive

material (Figure 3). Alundum grains were of the size

0.60 - 0.71 mm. The main constituent of erosive material

was aluminium oxide (minimum 99%). Other constitu-

ents were: silicon dioxide, iron oxide, calcium oxide and

sodium oxide.

A sample with tested coating was inclined by 45˚. The

tests were carried out at the temperature of 20˚C ± 2˚C

and the relative humidity of 65% ± 5%.

4. Investigation Results

The effect of weathering on the epoxy coating thickness

is presented in Figure 4. One may notice that coating

thickness decreased on average by 2% after a 3-year

ageing period. Swelling was observed only for unmodi-

fied coatings aged for 2 years. Addition of the nanofiller

favourably increased the coating structure tightness, what

prevented swelling of the nanofiller modified coatings.

Figure 3. Morphology of alundum particles.

= -x

- 0,6x + 135,9

= 0,9971

= -4,5x

+ 17,5x

- 17x + 180

= 1

= -2,2x + 184,8

= 0,8345

100

130

160

190

220

01234

Ageing time t, [y ears]

Thickness G, [



1 - unm odified

2 - m odified with n anoa l um ina

3 - m odified with n anosil i ca

Figure 4. Thickness of epoxy coatings subjected to natural

weathering conditions for 3 years.

Hardness characteristics (Figure 5) of epoxy coatings

reveal an essential effect of the modification with alu-

mina or silica nanoparticles on an increase of the unaged

epoxy coating hardness. After a 3-year natural weather-

ing only the alumina modified coatings showed increased

hardness.

Results of the surface topography measurements of

epoxy coatings reveal an essential effect of a 3-year

natural weathering on an increase of the coating surface

roughness (Figures 6 and 7). The highest Ra and Rz pa-

rameters—over the whole weathering period—were ob-

served for coatings modified with nanosilica. Reasons of

the increased surface roughness can be accounted for a

tendency of silica nanoparticles to form globules. Such

explanation is confirmed by the increase of Ra and Rz

parameters by over 100%. Positive effect of modification

on the surface roughness decrease was stated for epoxy

coatings with the alumina modified surface layer.

The carried out investigation on epoxy coatings, after

3 year natural weathering, showed a decrease of the re-

sistance to alundum particles impacts. Moreover, the

results of erosive wear investigation proved that modify-

cation of epoxy coatings with nanosilica caused a slight

= 3x

- 9,4x + 92,1

= 0,99 46

= 2,33 33x

- 9,5x

+ 7,16 67x + 9 0

= 1

= 3,25 x

- 12,05x + 95,95

= 0, 9993

100

01234

Ageing time t, [years]

Hardness H

1 - unm o di fi ed

2 - m odi fi ed wi th n anoa l umi na

3 - modified with nanosilica

Figure 5. Hardness of epoxy coatings after 3-year natural

weathering.

= 0, 0 15x

+ 0,035x + 0,405

= 0,985

= -0, 01x

+ 0, 1 36x + 0, 371

= 0,9722

= 0,0 25x + 0, 9 65

= 0, 4 181

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

0123

Ageing time t, [years]

Roughness parameter R

, [



1 - unmodi fied

2 - modi fied wi th na noal um i na

3 - modi fied wi th na nos il i ca

Figure 6. Effect of natural weathering on the Ra parameter

of epoxy coatings.

Assessment of the Erosive Wear Kinetics of Epoxy Coatings Modified with Nanofillers

decrease of their erosive wear resistance (by 5%). The

erosive wear resistance of epoxy coatings modified with

alumina was comparable with that of unmodified coat-

ings (Figure 8).

The surface layer of a nano-silica modified epoxy

coating showed the lowest erosive wear resistance (S)

after a 3-year natural weathering (Figure 9). Its erosive

wear resistance was over 12% lower compared to the

unmodified surface layer. The coating modified with

alumina nanoparticles showed the highest erosive wear

resistance—higher by 14% than the one of the unmodi-

fied coating.

Figures 10 and 11 present the effect of natural weather-

ing on a destruction of epoxy coating surface. As the result

of weathering, coating components lose cohesion with

epoxy resin and are released from coating surface layers.

5. Conclusions

In summary, it is concluded as follows:

1) Modification of the epoxy coating structure advan-

tageously affected the resistance to erosive particles ac-

tion of the coatings weathered naturally for 3 years only

= -0,1675x

+ 1, 2975x + 1,9525

= 0,9553

= 1,01x + 2,08

= 0,9986

= 0, 3075x

- 0,6115x + 5,673 5

= 0,9873

0,0

1,5

3,0

4,5

6,0

7,5

9,0

01234

Ageing time t, [y ears]

Roughness parameter Rz, [



1 - unmodi fied

2 - modifie d with nanoal um i na

3 - modifie d with nanosi l i ca

Figure 7. Effect of natural weathering on the Rz parameter

of epoxy coatings.

= -0,115x

+ 0,5x

- 0,665x + 1, 0 2

= 1

= -0, 15 17x

+ 0,67x

- 0,8 983x + 1,21

= 1

= -0,14x

+ 0,615x

- 0,865x + 1,28

= 1

0,3

0,5

0,7

0,9

1,1

1,3

0123

Ageing time t, [y ears]

The resistance to erosive wear S, [ kg/

m]

1 - unm odi fi ed

2 - m odi fi ed wi th n anoal umi na

3 - m odi fi ed wi th n anosi lica

Figure 8. Erosive wear resistance of unmodified and modi-

fied epoxy coatings.

= 0,0275 x

- 0, 2955x + 0 ,9795

= 1

= 0, 075x

- 0,465x + 1, 1 65

= 0,9984

= 0, 045x

- 0,407x + 1, 2 03

= 0,9974

0, 3

0, 5

0, 7

0, 9

1, 1

1, 3

01234

Ageing time t, [y ears]

The resist an ce t o er os ive wear S, [k g/



1 - unmodi fied

2 - modi fied with nanoa lum i na

3 - modi fied with nanosil ica

Figure 9. Erosive wear resistance of the surface layers of

unmodified and modified epoxy coatings.

Figure 10. Surface morphology of the epoxy coating modi-

fied with alumina nanoparticles: unaged (a) and subjected

to natural weathering conditions for 3 years (b).

Figure 11. Surface morphology of the epoxy coating modi-

fied with silica nanoparticles: unaged (a) and subjected to

natural weathering conditions for 3 years (b).

in the case of alumina nanoparticles application.

2) This is due to increased hardness of the nano-alu-

mina modified coatings and their lower surface rough-

ness compared to the unmodified coating.

3) Modification of epoxy coatings with nano-silica re-

sulted in lowering of the coating erosive wear resistance

during the whole 3-year weathering period. It resulted

from decreased hardness and increased surface roughness

of this type of coatings. Reasons of the increased surface

roughness of the nano-silica modified coatings can be

accounted for a tendency of silica nanoparticles to form

globules.

4) Moreover addition of a nanofiller, both silica and

Assessment of the Erosive Wear Kinetics of Epoxy Coatings Modified with Nanofillers

alumina, favourably increased the coating structure tight-

ness, what prevented humidity swelling of the nano-

filler modified coatings. Swelling was observed only in

the case of unmodified coatings after 2-year weathering

period.

5) Summarising, one can state that modification of

epoxy coatings with nanofillers resulted in an increased

in-service durability only in the case of modification with

alumina nanoparticles.

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